This mri image shows a superior view of the top of the brain, with the eyes visible at the top of the image. this is an example of a(n)

Which of the following must be true for a fossil organism to be useful as an index fossil? a. the fossil organism must be widespread geographically. b. the fossil organism must be abundant. c. the fossil organism must be limited to a short span of geologic time. d. all of the above.